Rear fluid reservior

This may sound dumb but I just bought my 2000 Sil. a couple of weeks ago and can't find my rear window washer fluid reservoir.

I don't think there is one. The rear washer is usually serviced by the washer fluid reservoir in the engine compartment with a separate line going all the way to the rear of the van.

thats right, and it takes forever for the fluid to get there.

I suspected so. After looking the front Res. appears to have two pumps mounted. One high / one low.
The rear sprayer worked when I got the car. The pump in the rear now makes noise but no fluid. I had run low in the front Res., below the high mounted pump, before it stopped. I have since refilled. Hopefully it just takes a long time to re-prime.

You might be able to fish a small wire in the nozzle to clear it maybe clogged, there is also a connection in the headliner inside the rear door.

You are correct there are two pumps, the upper for the rear and lower for the front.

When they get low, the rear pump is a little higher so it won't squirt to the back.

I had run low in the front Res., below the high mounted pump, before it stopped. I have since refilled. Hopefully it just takes a long time to re-prime.
Yes it can take awhile to get the new fluid to the back when you run low. I usually hold the spray lever for the back in short spurts until I get the fluid spraying on the rear window. I'm afraid to hold the lever down to prime the rear pump for too long in fear of burning out the pump.